1. Field of the Invention
The invention in general relates to the field of LTCC (low temperature cofired ceramic) circuitry.
2. Description of Related Art
An LTCC circuit is comprised of a plurality of ceramic layers with passive and/or active components and stacked together to form a module, with each ceramic layer containing thick film printed circuitry metallization, generally gold or silver. The ceramic layers include conductive vias for making electrical contact between layers and the stack of ceramic layers is cofired at a temperature high enough to sinter the layers, yet low enough so as prevent flowing or melting of the metallization. The result of the firing is to form a rigid monolithic structure.
LTCC circuits have high packing density, can be customized to meet desired applications, are cost effective, reliable and can be controlled with respect to dielectric values. The LTCC allows for integration of digital and RF, stripline and microstrip circuits in a single light weight 3-D package. Such LTCC circuits are used for high frequency applications in both military as well as commercial devices.
Often, after the LTCC module is cofired, it is desirable to add additional layers of metallization, in a certain pattern, to build up a base, on one or more surfaces, to which some component is to be attached. Such component may be a frame structure, a heat sink or an electrical connector block, by way of example.
Typically, the base to which the component is attached is made up of a plurality of metal layers to achieve a desired thickness. Each layer of the base is screened on, in the predetermined pattern and then fired at the cofiring temperature of around 850° C. This procedure of repeated firings of the LTCC module, however, at the elevated temperature, has an adverse affect on buried components such as resistors, capacitors and inductors which may change with these repeated firings. For example buried resistors may actually change their initial resistance value by more than 50% with repeated firings. Further, repeated high temperature firings may affect solderable layers and change their bondability.
In addition, some ceramic materials which comprise the ceramic layers will also change with repeated high temperature firings. As the ceramic material is heated to a temperature above its crystallization temperature, additional crystallization occurs. This crystallization can modify the dielectric constant and density of the ceramic.
It is an object of the present invention to minimize or eliminate the above described changes without degrading the performance of the metallization forming the base.